1. Field of the Invention
The present invention relates to the field of rotary kilns and means of thermally insulating it. More particularly, this invention relates to a rotary kiln having a hollow-brick insulating lining between the kiln shell and the work lining, used for pyro-processing materials such as cement, lime, lime sludge, clay, shale, refractory, bauxite, garbage, ores, fuels and minerals. Such kilns normally vary in diameter between 6 and 25 ft. and in length between 200 and 600 ft., and are supported along their length by pairs of support rollers and riding rings secured to the outer surface of the shell. In U.S. Pat. No. 4,344,596, Hjaeresen provides a detailed description of the kiln mechanics. In U.S. Pat. No. 4,200,469, Touborg provides a description of how rotary kilns are fired.
Conventional insulated rotary kilns used for pyro-processing a wide variety of materials, are commonly lined with two concentric layers of refractory brick or castable. The insulating layer is installed directly on the inner surface of the shell, and the dense or work layer is installed on top and concentric with the insulating layer. The bricks in the work layer are dense and of a heat resistant refractory composition such as magnesia, dolomite, alumina, or clay, which sufficiently resist the intense heat produced in the kiln. Heat resistant refractory materials are usually good heat conductors and relatively poor heat insulators. In the rotary kiln field, one of the main problems is to contain heat within the processing zone to prevent the fast overheating and destruction of the kiln shell. Another common problem is to protect the kiln shell against permanent damage when the work lining collapses. Due to the very high temperatures maintained inside the kiln and the high thermal conductivity of the work lining, part of the thermal energy inside the kiln is lost through the work lining and the kiln shell, thus requiring that the shell be thermally insulated. Besides the heat loss, which translates into additional fuel expenditure and additional combustion gas emissions, the temperature gradient between the inside hot face and the cold face of the refractory lining develops thermo-mechanical stress within the work lining. When said stress exceeds the ultimate strength of the brick, the lining fails. Therefore, it is important to use an insulating lining between the kiln shell and the work lining.
Rotary kilns are sometimes exposed to strong winds, snowstorms and rainstorms. When the hot kiln shell is suddenly quenched, it shrinks and the refractory lining is exposed to hoop stress that can cause its rupture. For such reason it is important to insulate the inside of the kiln shell in order to reduce its outside temperature.
Several patents propose ways to insulate the inside of the rotary kiln shell, behind the work lining. The prior art normally employs two concentric layers of bricks, or two concentric layers of castable or a brick layer concentric with an insulating refractory layer. Insulating materials are normally lightweight fiber, brick or castable with thermal conductivity below 10 B.t.u./hr./sq.ft./deg. F./in. thickness. In insulated rotary kilns the typical work lining thickness varies between 6 in. and 9 in., and the typical insulating lining thickness varies between 0.25 in. and 3 in. In order to accommodate the inner kiln shell curvature the brick shapes have different tapers that, when combined in the proper ratio, can line any kiln radius. Sometimes the back up insulating brick can be either tapered or straight since it is held in place by the work lining.
2. Description of the Prior Art
Many prior art patents cover the field of rotary kiln shell insulation between the work lining and the kiln shell. Prior art for rotary kiln shell insulation comprises a wide variety of dense or work linings installed on top of fiber refractory, steel cladded fiber refractory, solid insulating brick, solid insulating castable, solid fireclay brick, solid ceramic tiles, grooved brick and tiles, brick with recesses or pockets on the outside face, and bricks with pockets on the outside face filled with fiber. For example, in U.S. Pat. No. 1,410,729 Balz invented a dense brick attached to an insulating refractory by a tongue and groove system. In U.S. Pat. No. 1,701,287 Waite invented a static furnace wall construction containing air channels behind the work lining to insulate the furnace shell. In U.S. Pat. No. 1,622,431 Feigenbaum invented an air cooled static vertical incinerator. In U.S. Pat. No. 1,674,422 Allen invented an air cooled wall. In U.S. Pat. No. 1,688,321 Ser. No. 10/23/1928 Abott invented an air cooled furnace wall. In U.S. Pat. No. 2,641,205 Dolezal invented a cooling wall for heated chambers. In U.S. Pat. No. 5,695,329 Orcutt invented a way to insulate the kiln shell with fiber material inserted between the work lining and the kiln shell. In U.S. Pat. No. 1,936,635 Lee proposed a special-shape solid brick that, when put together, creates an air gap between parts of the brick and the kiln shell. In U.S. Pat. No. 2,230,141 Heuer proposed a dense brick cemented to a solid block of insulating brick or asbestos. In U.S. Pat. No. 2,635,865 Brumbaugh invented a layer of insulating concrete anchored to the kiln shell between the work lining and the shell. In U.S. Pat. No. 3,343,824 Schneider used multiple layers of asbestos sheets as an insulator between the work lining and the kiln shell. In U.S. Pat. No. 3,528,647 Hyde employed diatomaceous earth as an insulator between the shell and the work lining of a blast furnace, a vertical, non-rotary cylindrical furnace. In U.S. Pat. No. 4,289,479 Johnson utilized solid blocks of lightweight insulating material attached to the kiln shell as insulator. In U.S. Pat. No. 4,499,134 Whitely used steel cladded fiber as an insulator behind the work lining. In U.S. Pat. No. 4,582,742 Gilhart used blocks of high-temperature fiber insulation as a back up lining in a furnace. In U.S. Pat. No. 4,803,933 Carey invented a rotary kiln brick with recessed chambers filled with insulating pads on the outside face. In U.S. Pat. No. 5,033,959 Bernt proposed cladded insulating fiber behind the work lining. In U.S. Pat. No. 5,695,329 Orcutt invented a way to insulate the rotary kiln shell with layers of reinforced insulating fiber installed behind the work lining.
The prior art of insulating rotary kilns presents some specific problems such as: asbestos is practically banned as an industrial thermal insulator because of its well-proven carcinogenic properties; ceramic fibers shrink when heated above 2000xc2x0 F., causing the service lining to collapse; solid insulating and semi-insulating bricks have relatively low compressive strength, usually lower than 3,000 p.s.i.; solid insulating and semi-insulating bricks shrink when overheated, which could cause the work lining to collapse; solid fireclay or alumina brick and tile have good compressive strength and good thermal stability but their thermal conductivity is higher than insulating fiber""s and insulating brick""s; bricks with air gaps or pockets on the outside face help reduce heat transfer from the kiln processing zone to the shell, but they have reduced contact with the shell, a mechanical disadvantage over solid brick alternatives; bricks with air gaps or pockets on the outside face, filled with insulating materials, help reduce heat transfer from the kiln processing zone to the shell, but these bricks have reduced contact with the shell, a mechanical disadvantage over solid brick alternatives; dual bricks, comprised of a dense brick cemented or sintered to a lightweight brick, have the disadvantage of the differential thermal expansion coefficient between the two materials. Such difference creates a shear plane within the brick that causes it to crack; forced air or convective air channels between the work lining and the shell are not applicable to rotary kilns due to their constantly turning motion and their length.
One of the main disadvantages of most prior art patents mentioned herein is the low refractoriness and the relatively low strength of the insulating lining. When the work lining fails, the insulating lining is immediately exposed to temperatures above its strength and refractoriness limit, causing it to fail. Another common problem with the prior art is the compromise between thermal conductivity and material strength. It is well known in the science of ceramics that the thermal conductivity of a porous refractory decreases with its bulk density, but so does its mechanical strength. Consequently, the more the thermal conductivity of a porous solid brick is reduced, the lower its mechanical strength. The same principle applies to dense bricks.
With this invention I have eliminated many of the disadvantages and problems of the prior art. The hollow brick is mechanically stronger than insulating brick, insulating castables, and fibers, with minimum cold compressive strength of 4,000 p.s.i. Moreover, the hollow brick is more refractory than insulating fiber, insulating castables, and insulating brick, with minimum pyrometric cone equivalent of 32.5 in the Orton scale. Unlike refractory fibers, the hollow brick can be used as a permanent lining, therefore allowing the replacement of the work lining without disturbing the insulating lining. The hollow brick material does not contain asbestos or any other hazardous substance in its composition. The hollow brick, not having recesses, grooves, or pockets on the outside face, make full contact with the kiln shell, resulting in mechanically more stable linings. The hollow brick utilizes still air, one of the best insulating substances, as insulating medium. The hollow brick allows kiln designers to change the thermal conductivity of the lining without changing its mechanical strength, refractoriness, and lining thickness, one of the major problems encountered with the prior art.
The present invention consists of a rotary kiln for pyro-processing a wide variety of materials and includes an elongated, cylindrical steel shell through which the material transverses for processing. The kiln shell is rotated by conventional mechanical structure, well described in some of the patents referred to herein, and includes a feed end and an opposite discharge end. Furthermore, the shell includes an open processing zone extending from the feed end to the discharge end, through which the material travels and the high temperature gases circulate. The kiln shell includes an inner cylindrical surface circumjacent the open processing zone. An insulating refractory lining is disposed contiguous to the inner cylindrical surface of the shell and annularly to the longitudinal axis. The lining is comprised of adjacently independent rings of hollow bricks, held against the kiln shell by their tapered design. In another variant, the hollow bricks have no taper and are held in place by the work lining.
It is an objective of the present invention to provide a rotary kiln with an insulating refractory consisting of hollow bricks. It is another objective of the present invention to provide an insulating lining for a rotary kiln in which the insulating bricks are provided with cells or cores running parallel to the brick outside face, in the radial or axial direction, to profit from the insulating properties of still air. Still another objective of the present invention is to provide a lining for a kiln capable of supporting a layer of dense refractory material on top; without collapsing. It is yet another objective of this invention to provide an insulating lining for a rotary kiln capable of withstanding process temperatures for sufficient time to allow the kiln operator to shut the kiln down without permanent damage to the steel shell, in the event the work lining fails.